This invention relates to circuits for operating fluorescent lamps and, more particularly, to an improved dimming circuit for fluorescent lamps.
With the growing need for saving energy in the home, the significantly greater efficiency of fluorescent lamps, in terms of lumens per watt, as compared to the more commonly used incandescent lamps has spurred engineering development of improved methods for placing fluorescent lamps into more wide-spread residential use. One approach toward accomplishing this end has been to search for improved retrofit systems for the existing incandescent lamp fixtures. More specifically, an object has been to provide an improved incandescent-to-fluorescent adapter having a simplified, lightweight and compact design of reduced cost. Typically, such an adapter has consisted of a ballast, a starter for preheat operation, a fluorescent lamp, and mounting and support hardward. The base of the adapter contains a standard screw-type base for mating with the receptacle of an incandescent fixture. In cases where it is desired to retrofit the standard three-way incandescent fixture having a four-position switch (off-low-medium-high) for operating two filament lamps, such as the 50-100-150 watt type, it is necessary to provide a multiple level dimming circuit for a fluorescent lamp. In the past, such retrofit systems were operated at different light output levels either by using two separate preheat circuits or by using a solid-state dimming ballast. The first method requires two chokes, two starters and two lamps. Such systems tend to be comparatively costly and can add too much weight to the top of the fixture, whereby some small fixtures could tip over. The second method, which uses a solid state ballast, is also quite costly, and semi-conductor component failures can be comparatively frequent. Also, many solid-state ballasts which operate at high frequencies produce RF interference.